A time-of-flight (TOF) method for measuring a distance from an object to a distance sensor by emitting a pulsed light from a light source and receiving reflected light from the object using the distance sensor is known.
A distance measurement device based on a TOF method is described in Patent Literatures 1 and 2 below. The device described in Patent Literature 1 emits pulsed light from a light source, acquires, from a distance sensor, a signal having a value corresponding to an amount of charges generated by a photodiode of the distance sensor in each of sub-periods having different lengths in one frame period, and calculates a distance based on a best signal among a plurality of acquired signals.
The device described in Patent Literature 2 has a configuration for widening an effective dynamic range of a distance sensor. Specifically, this device emits pulsed light from a light source, accumulates charges generated by a photodiode of the distance sensor in a capacitor, resets a voltage generated in the capacitor when the voltage reaches a saturation voltage, and calculates a distance based on the number of resets and a last voltage generated in the capacitor.
Further, background light is included in light to be incident on the distance sensor, in addition to signal light generated by the pulsed light emitted from the light source being reflected from an object. The distance measurement devices described in Patent Literatures 1 and 2 do not basically consider removal of noise components, such as a background light component included in the signal from the distance sensor.
Another distance measurement device based on a TOF method is described in Patent Literature 3 below. The device described in Patent Literature 3 emits pulsed light from a light source, acquires a signal from a distance sensor in a first frame period, and acquires a signal from the distance sensor with the light source being in a non-emission state in a second frame period. The frame period is a time period from resetting charges of an accumulation region which accumulates charges generated by a photosensitive region of the sensor to next resetting of the charges of the accumulation region. This device removes noise components in a signal by subtracting the signal of the second frame period from the signal of the first frame period. The device calculates the distance based on the signal from which the noise components have been removed in this way.